(a) Field of the Invention
The present invention relates to a method for growing a single-crystal silicon ingot based on the Czochralski method, and more particularly to an apparatus and method for growing the single-crystal silicon ingot while maintaining a uniform growing condition and cooling condition in a radial direction of the ingot; and the single-crystal silicon ingot and wafers produced therefrom, having uniform vacancy defects in a radial direction.
(b) Description of the Related Art
The voltage-resistance characteristic of an oxide layer of a semiconductor device fabricated from a single-crystal silicon substrate can be represented by the tolerable voltage and time before the silicon oxide layer formed on the silicon wafer is destroyed and the insulating property of the silicon oxide layer disappears. It is well known that semiconductor device characteristics, including the voltage-resistance characteristic of an oxide layer, change according to the method of fabricating the single-crystal silicon substrate.
For example, a silicon substrate made of single-crystal silicon grown by the Czochralski (CZ) method is inferior to that of single-crystal silicon fabricated by the float zone method or a substrate fabricated by silicon epitaxial growth on a wafer prepared from single-crystal silicon grown by the Czochralski (CZ) method, in view of the voltage-resistance characteristics of an oxide layer. Yet, despite its drawbacks, the CZ method is the most common technique used for manufacturing a single-crystal ingot for silicon wafers, which are used for electronic devices such as semiconductors. The CZ method is less expensive and has been widely used to prepare semiconductor materials for large-scale integration (LSI), and does yield materials having good mechanical and electrical characteristics.
As electronic devices become more highly integrated and minimized, the gate oxide layer is required to have improved reliability. Since the voltage-resistance characteristic of an oxide layer is a primary material characteristic determining the reliability of the device, there is a strong need for a single-crystal silicon production technique capable of securing the superior voltage-resistance characteristic of an oxide layer, while still using the Czochralski method.
In the CZ method, a seed crystal is dipped into a silicon melt and then slowly pulled away from the melt, growing the ingot. The growth of the ingot is carried out through several processes.
First, a necking process is carried out to form a slender and long neck portion from the seed crystal. Second, a shouldering process is carried out to grow the crystal radially to obtain a target diameter. Third, a body-growing process is performed to obtain a crystal having a uniform diameter. Here, a part of the body of the ingot is made into a wafer.
After the body growing process, a tailing process is performed to slowly decrease the diameter of the ingot and separate the ingot from the silicon melt.
These processes for growing the crystal ingot are carried out in a space called a “hot zone,” which includes a heater inside a grower, and other heat insulating components.
The defect characteristics of a single crystal sensitively depend on the growing and cooling conditions of the crystal, and there has been much effort to control the species and distribution of the defects formed during the growth (so-called “growth defects”) by adjusting the thermal environment near a growth interface.
The growth defects are divided into categories: vacancy-type defects and interstitial-type defects. They are caused by an agglomeration of the vacancy point defects or interstitial point defects, which starts from being more than an equilibrium concentration.
The Voronkov theory, introduced in “The Mechanism of Swirl Defects Formation in Silicon”, Journal of Crystal Growth 59 (1982) 625, teaches that growth defect formation is closely related to a value of V/G wherein V is a growing speed and G is a temperature gradient in the crystal near the crystal growing interface.
A vacancy-type defect is formed when the value of V/G exceeds a critical value, and an interstitial-type defect is formed when the value of V/G is lower than the critical value. Thus, the species, size, and density of the defects existing in the crystal are influenced by the pulling speed when the crystal grows in a given hot zone.
FIG. 1 is a cross sectional view of a crystal ingot growing by the CZ method, illustrating a typical defect distribution developed while controlling the growing speed such that an oxidation-induced staking fault (OISF) ring 200 is located around the circumferential edge of the ingot in a typical growth environment.
FIG. 2 is an image taken by minority carrier life time (MCLT) scanning on a cross-sectional surface of the single-crystal silicon ingot grown using the Czochralski CZ method in a typical growth environment.
As shown in FIG. 2, micro-vacancy defects, such as direct surface oxide defects (DSOD), exist around the exterior circumferential part of the ingot, since the cooling speed at the ingot exterior is faster than that at the interior of the ingot. Furthermore, coarse-vacancy defects, such as crystal originated particles (COP) or flow pattern defects (FPD), exist in the ingot interior since the value of G increases as one moves from the center of the ingot to the circumferential edge of the ingot.
In a single-crystal silicon grown by the Czochralski method, a primary factor decreasing the voltage-resistance characteristic of the oxide layer is micro-defects having a size smaller than a critical value, which are formed by the vacancy defects introduced into the crystal while pulling the single-crystal silicon ingot.
In order to improve the voltage-resistance characteristic of the oxide layer, the present invention controls the temperature gradient (G) in an axial (vertical) direction at a solid-liquid interface of the crystal and/or the initial concentration of the point defect when the silicon melt solidifies into the single-crystal silicon.
In another approach, the cooling speed of the crystal is controlled in a temperature range between the solidification temperature and about 1000°, in which the nuclei of defects is are formed and grows during the thermal processing history while the molten silicon is solidified into the single-crystal silicon. In this manner the silicon interstitials or vacancies diffuse forward to the side surfaces of the ingot or are accelerated to recombine mutually, so that it is possible to suppress the super-saturation of the interstitial or vacancy below the critical value at which the agglomeration occurs.
The introduced vacancy defects are grown through the diffusion, nuclei generation, and solidification according to the thermal history distribution of the hot zone. There has been effort to remove or reduce crystal originated particles (COP), which adversely affect the voltage-resistance characteristics of the single crystal silicon.
As shown in FIG. 1, if the ingot is grown in the pulling direction such that the oxidation-induced stacking fault ring exists at the typical hot zone, coarse vacancy defects are formed at the center part of the ingot due to heat accumulation and slow cooling effects. Since the cooling speed in the region between the exterior of the center part and the interior of the oxidation-induced staking fault ring is faster than that at the center part of the ingot, micro-vacancy defects are formed, and the vacancy defects are not uniformly distributed in the radial direction.
Also, the micro-vacancy defects formed due to fast cooling deteriorate the voltage-resistance characteristic of the oxide layer, and this occurs around the exterior circumferential part of the wafer at which the rate of cooling is relatively high.
Accordingly, in order to reduce these vacancy defects that cause problems in semiconductor devices made with single-crystal silicon wafers, it is required to decrease the pulling speed of the growing ingot. However, lowering the pulling speed causes a loss in productivity, and excessive reduction of the pulling speed is likely to, increase the formation of interstitial defects. A different approach is needed.